Huanglongbing (HLB), also known as citrus vein phloem degeneration (CVPD), citrus greening disease, yellow shoot disease (translated from Chinese huang-lunpin), likubin in Taiwan (translated from Chinese as Immediate Withering Disease), leaf mottle yellows in the Philippines, and citrus dieback in India, is probably the worst disease of citrus caused by a vectored pathogen. The causative agent is a motile bacterium, Candidatus Liberibacter spp., which is transmitted by Asian citrus psyllids (Sternorrhyncha: Psyllidae), also known as Diaphorina citri or, in Africa, by Tryoza erytreae, the African citrus psyllid.
Distribution of HLB is primarily in tropical and subtropical Asia. It has been reported in all citrus-growing regions in Asia. The disease has affected crops in China, Taiwan, India, Sri Lanka, Malaysia, Indonesia, Myanmar, the Philippines, Pakistan, Thailand, the Ryukyu Islands, Nepal, Réunion, Mauritius, and Afghanistan. Areas outside Asia such as Saudi Arabia, Brazil and Florida in the U.S. have also reported the disease.
Although existing insecticidal and repellent compositions may be useful in controlling HLB, the safety of these compositions has been questioned as many of these compositions are excessively toxic to other organisms in the ecosystem. In addition, many of these compositions are extraordinarily long-lived, and persist within the environment to which they are applied almost indefinitely. Moreover, many insect species have evolved resistance to many of the known insecticidal and repellent compositions. Thus, a need exists for a relatively non-toxic, shorter-lived, effective repellent composition, such as a biological repellent composition.
It is long known in Vietnam that guava grown in proximity to or intercropped with citrus has a protectant or repellant effect against the Asian citrus psyllid. Guavas are plants in the myrtle family (Myrtaceae) genus Psidium, which contains about 100 species of tropical shrubs and small trees. The most frequently-encountered species, and the one often simply referred to as “the guava”, is the Apple Guava (Psidium guajava).
The protective effect of interplanting guava and citrus is likely due to volatiles produced from the guava leaves because the protective effect is present year round. Although fifty-seven components including 27 terpenes (or sesquiterpenes) along with 14 alcohols and 4 esters have been identified in guava leaf oil using GC-MS obtained from a hydrodistillation of guava leaves, the exact mechanism underlying the protective effect of guava is not known. A recent report (Rouseff et al., Sulfur Volatiles in Guava (Psidium guajava L.) Leaves: Possible Defense Mechanism, J. Agric. Food Chem, 56:8905-10, 2008) suggests that sulfur volatiles such as dimethyl disulfide are responsible for the repulsive effect of guava, not the major volatiles such as β-caryophyllene since the latter is also present in citrus. However, sulfur volatiles contained in guava leaves are emitted at tiny levels and only for a period of about 30 minutes after wounding, indicating that these compounds have nothing or little to do with what is observed in citrus-guava intercroppings in Vietnam. On the other hand, β-caryophyllene is present in undamaged citrus leaves at concentrations generally below 0.2% of total volatiles, while it represents usually more than 50% of total volatiles emitted by guava leaves.
There remains a need, for controlling HLB in citrus plants, and the present invention provides a novel solution to this problem.